Rubber accelerator



RUBBER ACCELERATOR Robert C. Kinstler, Dunellen, N. J., Robert S. Barker,

Port Washington, N. Y., and Gosta Brunnstrom, Somerville, N. .L, assignors to American Cyanamid Company, New York, N. Y., a corporation of Maine No Drawing. Application August 1, 1956 Serial No. 601,347

9 Claims. (Cl. 260-3065) This invention relates to a dust free composition comprising a rubber vulcanization accelerator. More particularly, it relates to a pelletized composition comprising benzothiazolyldisulfide, and a method of preparation.

Solid rubber compounding ingredients, for instance, vulcanization accelerators such as benzothiazolyldisulfide, are generally prepared as fine powders. Finely powdered benzothiazolyldisulfide, however, when incorporated into rubber by milling leaves much to be desired. For

example, it sticks and cakes on the mill rolls and is not readily dispersed in the rubber. In addition, it is extremely dusty so that the air surrounding the mill soon becomes contaminated. This results in a loss of material as well as undesirable working conditions for those required to handle the material.

These disadvantages could be greatly reduced if finely powdered benzothiazolyldisulfide could be formed into aggregates or pellets. It has been suggested that this be accomplished by agitating an aqueous suspension comprising the powder and a waxy material at a temperature above the melting point of the latter, whereby it is alleged that agglomerates of benzothiazolyldisulfide will form through the binding action of the waxy material. It has also been suggested that such pellets might be formed by compressing a mixture of benzothiazolyldisulfide, arubber hydrocarbon and a hardening agent into a'solid mass which could then be subdivided into pellets of the desired size.

Commercially acceptable pelletized benzothiazolyldisulfide must meet two requirements which appear to be conflicting. First, the particles comprising each pellet should cohere sufficiently to withstand shipment and handling without breaking down to give dustyfines. Second, when subjected to the shearing forces and mill temperatures during mastication on the rubber mill, the pellets should disintegrate to give a uniform and fine. dispersion of benzothiazolyldisulfide particles throughout the rubber. The pellets should be readily wetted by the rubber so that they do not fall out of the rubber on the under side of an open mill roll. The pellets must not be so hard that they pop from the nip of an open mill roll. It is possible to make pellets soft enough to be broken by the milling shear yet hard enough to pop or fly from the roll nip. 7

It is a relatively simple matter to make pelletized benzothiazolyldisulfide. It is another matter to form pellets which disperse well in rubber. High molecular Weight plastics, resins, glue, and the like will bind benzothiazolyldisulfide into pellets of high mechanical strength such as to satisfy the shipment and handling requirement. On incorporation into rubber, however, such pelletsare broken down only to the extent of roll clearance and are not materially affected by the shear forces in the rubber. Such binders, moreover, are neithersoluble in rubber nor do they melt or soften at milling temperatures, the presence of whichcharacteristics provide for good dispersion. i

- It is a principal object of, this inventionto provide 5' employed as a solution of an alkali salt.

2,840,564 Patented June 24, 19 58 benzothiazolyldisulfide as pellets which comply with the requirements set forth above. It is particularly an object of this'invention'to provide pellets of benzothiazolyldisulfide which are characterized by the quality of excellent dispersibility when incorporated into rubber. It is a further object of this invention to provide a method for producing such pellets. It is a still further object of this invention to provide such a method which is capable of consistently reproducing such an excellent dispersing pelletized product with a minimum of control and supervision.

In accordance with the present invention, theseobjects have been met in a surprisingly effective manner. In general, pelletized benzothiazolyldisulfide having the desired characteristics is prepared by forming an aqueous slurry comprising finely divided benzothiazolyldisulfide and a small amount of aminothiophenol. The aqueous slurry is then heated and agitated to provide intimate contact for agglomeration. After agglomerates of desired size have been formed, the slurry is cooled and resultant pellets separated.

The aqueous benzothiazolyldisulfide slurry subjected to the pelletizing process of this invention may be prepared by oxidizing an aqueous solution of sodium mercaptobenzothiazole. This latter solution may result from purification of an impure mercaptobenzothiazole melt by digesting with a deficiency of caustic thereby leaving tarry by-products and part of the mercaptobenzothiazole out of solution. Alternatively, the purification may comprise steam stripping animpure mercaptobenzothiazolemelt as taught in' U. S. Patent No. 2,658,864 and then digesting the resultant mercaptobenzothiazole in aqueous solution with an excess of caustic followed by a selective precipitation of mercaptobenzothiazole-free tarry impurities according to U. S. Patent No. 2,730,528.

The benzothiazolyldisulfide slurry may also be prepared by isolating the disulfide as prepared by either of the above processes and reslurrying the dry, powdered accelerator in water. Additionally, it may also be prepared by precipitating and separating free mercaptobenzothiazole from a solution of the sodium salt thereof, redissolving it in caustic and then oxidizing it to disulfide. Because the formation of pellets according to the process of this invention, unlike some processes, is not dependent on the absence or presence of impurities and/ or by-products generally found in the tarry settler heels always present in most mercaptobenzothiazole purification procedures, it is immaterial how the benzothiazolyldisulfide slurry is prepared. I

The density of the aqueous benzothiazolyldisulfide slurry employed in the instant process is limited principally by apparatus limitations. solids content is much in excess of about 30% is too pasty and is diflicult to agitate and pump in a satisfactory manner. Too concentrated slurries, moreover, tend to reduce the control of pellet size which is important. Accordingly, the solids content of the slurry should not be much in excess of 30% and usually will be from about The agglomerating agent, i. e., aminothiophenol, is best In practice, it will usually be the sodium or potassium salt. However, other salts such as the ammonium or lithium salts may also be employed. The amount of real aminothiophenol employed may be quite widely varied. Generally, it will be the practice to use at least about 1% on the weight of the benzothiazolyldisulfide. 7 than this even ranging to as high as about 25%. More than about 25%, however, tends to produce/a pelletized product which is soft and difficult to handle. The amount of aminothiophenol employed, therefore, will-usually, be

A slurry of which the Normally, 'it willbe greater stances in as short a period as 15-30 minutes.

less than about 25% by weight of the benzothiazolyldisulfide, generally in the range of about -15 After the slurry comprising benzothiazolyldisulfide and arninothiophenol has been prepared, it is subjected to heating- Preferably, the temperature is raised to at least abo t '75 c. and preferably 5-100 ,c.\ Agglo neration does not appear to progress too well much below 85 C.

smaller and more uniform the size of the subsequently recovered pellets. When agglomeration has reached the.

desired point, the slurry is cooled and resultant pellets separated. i

i aminothiophenol should cause agglomeration and result'inthe formation of pellets of benzothiazolyldisulfidewhich are characterized by unusually excellent dis-' persibility whereno pellets of any description are formed in its absence is not known. Nor is there anydesire to limit this invention by any explanation of this action which would be at the most'a mere supposition. It is clearly evident, however, that the action is not one of mechanical binding ofithe particles such as would apparently occur, for instance, when using Waxy binders as has been suggested in the prior art.

In accordance with the above-described procedure, the pH of the aqueous benzothiazolyldisulfide mixture prior to heating is about 8-8.5. In prior art pelletizing procedurcs in which a waxy material is suggested as the agglomerating agent, it is claimed to be distinctly advantageous to conduct the heating and agglomerating step at a neutral or acid pH. Prior to heating, therefore, and preferably before addition of the waxy material, the pH is adjusted as by the addition of hydrochloric acid to 5-7 or even lower. In the. present process, however, there is no such restriction. Because of the unusual agglomerating characteristics of aminothiophenol itis possible to conduct agglomeration at either acid or alkaline conditions.

By theprocess as described to this point, heating andv agglomerating must be conducted for a matter of hours before excellent .d ispersible pellets of the desired size are. obtained regardless of whether'th'e pH is acid or alkaline. This is not. wholly satisfactory inasmuch as equipment is tied up and production decreased by the prolonged agglomeration step. As an added feature and a preferred embodiment of this invention, however, it has been surprisingly discovered that the period of agglomeration can be materially reduced by acombination heatingpH adjustment step. i 1 l As already noted, the pH of the'benzothiaz olyldisulfide mixture. is'alltaline when initially prepared. According to this invention, this alkaline .mixtureis heated to greater than about. 85 C. and, preferably .to boiling, and maintamed there for a period of time ranging up to anlhourbutlgenerally -30 minutes. The pH .ofthe mixture, preferably without cooling, is then adjusted to neutral or acidas by the addition of acetic, hydrochlo iic sul furic or the like acids. .The adjusted pH will usually be about 5.0-5.5 but may be as lowas 4. Lower pHs,. however. are not referred becauseof foaming and corrosion problems. The elevated temperature is then maintained while agitating the mixture. Where agglomeration'in the absence of this unique heating and pH adjustment step may require as much as six hours, similar results'can now be surpr singly obtained in .l-2 hours and I Although the described heating-pH. adjustment step greatly reduces the agglomerating time nevertheless, the n time is not consistently reproducibleyranging from about l5 minutes to 2 hours. -In accordance with this inventron, however, itha s beenfurther surprisingly' found that if sulfur. is added to .1 mixture; the agglomeration at in many in- 4 the'acid pH can be consistently conducted in the unusual and heretofore unheard of time of l0-l5 minutes. Why elemental sulfur should have this effect on the process is not understood, but about 3-10% on the Weight of the benzothiazolyldisulfide has been found to stabilize at 10-15 minutes, an otherwiseerratic agglomeration period. The use of a greater amount of sulfur does not provide for any added advantage while lesser amounts do not ppear o r s t n o i t nt y s ort asslsm r p ods. The sulfur mayjbe added at any time prior to heating at the acid pH preferably as a fine powder but will usually be added before heatingon the alkaline side.

Other materials may also be added to the slurry so as to be present in the final pellets. Thus, it is highly desirable to have present in the pellets a fatty acid, such as stearic acid or a metal salt "thereof, to aid in dispersion of the pellets in rubber. Fatty acids have heretofore been ,7 suggested as agglomeratingagents in proposed pelletizing procedures. lnltheipreseiit invention, however, the presence of a fatty acid initlie' aqueous slurry undergoing treatment is not intended o-ca'usej agglomeration. It is added to the slurry so'th'at it will be' pr es'ent in the finalpellcts to serve as a lubricant therefor when dispersed inrubben.

The amount of real stearic acid required for lubricating purposes will range from about 5-15 but will generally beabout 842%. 7 Since. it does not effect pelletization, it may be added at any stage of the process. As a practical matter, however, it is desirable to add it to the aque-. ous slurry along with the other components.

Pellet size is related to intensity of agitation. The more violent the agitation, the smaller the pellet size. The intensity of agitation should preferably be uniform so as to' produce pellets of a uniform size range. Pellet size should preferably be larger than about" 100 mesh since? smaller pellets tend to be dusty. At the same time, the size should preferably not be greater than about 5 mesh since larger sizes tend to be difficult to disperse in Throughout .the examples the following abbreviations will be used:

BTS+ saz9t iaz l l wl d- ATP-aminothiophenol'.

V EXAMPLE 1 To a slurry comprising 25 of high purity MBTS' in 500parts of water isadded a solution of sodium ATP containing 5 parts of'ATl. "l'hisalkaline mixture at a pH of 8.5-9.0 is then heated to -l00 C. and agitated until pelletsf orm. The slurry is then cooled to 65 C. by adding cold water and the pelletiz ed product, about 10-16 mesh insize, separated by screeningand dried.

EXAMPLE 2 To 200 parts of an aqueous slurry containing parts of MBTS is added 7 parts real of ATP dissolved in dilute sodium hydroxide solution. The pH of the resulting mixture is, adjusted to about 6.5 and the slurry heated at 98-99 withagitation for 5 hours. The slurryis then cooled, theseparated washed and dried. The product pellets larger 100 mesh.

EXAMPLE 3 To a slurrytof '25 p'artsof high purity .MBTSjn 515 To 2440 parts of an aqueous slurry containing 100 parts of high purity MBTS is added a mixture of 5 parts of 200 mesh sulfur dispersed in a solution of sodium stearate containing 10 parts of stearic acid. While stirring the resulting mixture, 30 .parts of a solution of sodium ATP containing 4 parts of ATP is added and the whole heated to boiling for minutes. Dilute acetic acid is then added to bring the pH from 8.5 to 5.0-5.5. Upon additional heating at the boil and stirring for 15 minutes the slurry agglomerated into tiny pellets or beads. Cold water is then added to drop the temperature to 70 C. and the product separated by screening. The product comprises pellets of about mesh in size.

EXAMPLE 5 Four samples of an aqueous MBTS slurry, each containing the components specified in Table I, are heated to the boil for 15 minutes and then adjusted to a pH of 4. Each adjusted sample is then agitated and heated at boiling for sufficient time to give pellets of a size suitable for dispersing in rubber, then cooled and the pellets separated. The times required to obtain such pellets are indicated in Table I.

Table I Component parts/100 parts MBTS Agglomeration Sample time after pH adjustment Stearic ATP Sulfur (min) Acid 10 4 5 l5 l0 3 5 l5 l0 4 5 8 l5 6. 5 O 120 To illustrate the excellent dispersing property of the pellets produced according to this invention, the following example is conducted in which the pellets of this invention as well as commercially available MBTS pellets are incorporated into rubber.

EXAMPLE 6 The products listed in Table II are dispersed in natural rubber as accelerators by mixing according to a standard procedure on a two roll'mill to make a master batch. After the accelerator has been incorporated into the rubber, the resultant mix is sheeted out and examined by transmitted light for the presence of particles large enough to be distinguished by the naked eye. The fewer of such particles, the better is the dispersion. Results appear Example 7 further illustrates the agglomerating action of aminothiophenol by demonstrating that pellets are not obtained in its absence.

EXAMPLE 7 To 1200 parts of an aqueous slurry containing parts of MBTS is added a sodium stearate solution containing 10 parts of stearic acid. The mixture is stirred and dilute acetic acid added to reduce the pH to 5.0-5.5. It is then heated to and held at 98100 C. for 6 hours while stirring. There is no evidence of agglomeration of the fine particles. The slurry is quenched with cold water and filtered to yield a fine powder similar to the starting material.

Example 8 illustrates the addition of stearic acid as the lubricant after agglomeration is subsequently complete.

EXAMPLE 8 The procedure of Example 2 is repeated except that after agglomeration is complete but before cooling the slurry, 9 parts of stearic acid are added and the slurry agitated for an additional 5 minutes. The slurry is then cooled and the pellets separated. The pellets so obtained have better dispersion characteristics than those of Example 2.

We claim:

l. A method of pelletizing benzothiazolyldisulfide which comprises: agitating an aqueous slurry comprising benzothiazolyldisulfide and aminothiophenol at an elevated temperature for sufiicient time to agglomerate the benzothiazolyldisulfide, and separating resultant pellets.

2. A method according to claim 1 in which the pH of the slurry is adjusted to less than about 7 prior to agitating at an elevated temperature.

3. A method according to claim 1 in which the slurry is maintained at greater than about 85 C. for a short period, the pH adjusted to less than about 7, and the eledrogen ion concentration to an acid pH not less than about 4; agitating the slurry and maintaining the temperature thereof at greater than about C. for an additional period suflicient to form agglomerates; cooling the slurry and separating pellets.

6. A method according to claim 5 in which the pH is adjusted to 5.0-5.5.

7. A method according to claim 5 in which the aminothiophenol is added to the aqueous slurry as a solution of an alkali metal salt.

8. A method according to claim 5 in which about 5-15 stearic acid is present in the aqueous slurry when subjected to heating after pH adjustment.

9. pelletized benzothiazolyldisulfide composition prepared according to the method of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 2,598,229 Creed May 27, 1952 2,598,319 Throdahl May 27, 1952 2,762,855 Creed Sept. 11, 1956 UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No. 2,840,564 June 24, 1958 Robert Cu Kinstler et all It is herebfi certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 66, for "200" read 2000 column 5, Table II, under the heading "Accelerator", lines 2 and 3 thereof, for "Thiofide S (Monsanto pelletized MBTS)" read w Commercial MBTS as pellets same table, same column, lines 4 and 5 thereof, for "Rodform Altax (Naugatuck MBTS)" read Commercial ABTS as rods -o Signed and sealed this 9th day of December 1958,,

(SEAL) Attest:

KARL H1, AXLINE ROBERT C. WATSON Commissioner of Patent Attesting Oflicer 

1. A METHOD OF PELETIZING BENZOTHIAZOLYDISULDIDE WHICH COMPRISES: AGITATING AN AQUEOUS SLURRY COMPRISING BENZOTHIAZOLYLDISULFIDE AND AMINOTHIOPHENOL AT AN ELEVATED TEMPERATURE FOR SUFFICIENT TIE TO AGGLOMERATE THE BENZOTHIAZOLYLDISSULFIDE, AND SEPARATING RESULTANT PELLETS. 